POWER SYSTEMS MCQS SET 17

POWER SYSTEMS MCQS SET 17

1․ Proximity effect is more in case of
a.  cables      b. over head lines     c. same in both cases     
d. none
Explanation::
Proximity effect occurs due to current in mutual conductors. The effective area of current flowing path is reduced because of non uniform flux linkage between the two adjacent conductors. Proximity effect is more in case of power cables, because the distance between the conductors is small. This effect is negligible in case of over head transmission lines, because the distance between the conductors is larger.
 
2․ Proximity effect depends on
a. frequency            b. distance between the conductors
c. relative permeability              d. all of the above
Explanation::
Proximity effect occurs due to current in mutual conductors. The effective area of current flowing path is reduced because of non uniform flux linkage between the two adjacent conductors. Proximity effect depends on 1. Frequency 2. Conductivity 3. Relative permeability 4. Distance between the conductors
 
3․ Skin effective is more in case of
a. communication lines         b. power lines        
c.  same in both cases           d. none of the above
Explanation::
Skin effect ∝ 1/skin depth Skin effect is inversely proportional to the skin depth. skin depth = 1/√(πfµσ) Where, f = frequency µ = permeability σ = conductivity In case of communication line frequency is higher. So that skin depth is very small and hence skin effect is larger. In case of power lines frequency is small and hence skin depth is larger, so that skin effect can be neglected.
 
4․ Apart from the skin effect the non uniformity of the current distribution is also caused by
a. bundled conductors                b. ferranti effect
c. proximity effect                        d. all of the above
Explanation::
Proximity effect occurs due to current in mutual conductors. The effective area of current flowing path is reduced because of non uniform flux linkage between the two adjacent conductors. Proximity effect is more in case of power cables, because the distance between the conductors is small. This effect is negligible in case of over head transmission lines, because the distance between the conductors is larger.
 
5․ Transmission lines are transposed to reduce
a. ferranti effect      b. skin effect         c. proximity effect
d. interference with neighbouring communication lines
Explanation::
If power line and telecommunication lines are running close to each other, the current flowing in the power line produces magnetic flux linkage with the communication line conductor induces an emf in the telecommunication line conductor. This is called electromagnetic induction. Similarly due to earth effect electric field is produced by the charges of the earth induces a voltage in between the conductors of the telecommunication lines. This is called electro static induction. This both electromagnetic induction and electro static induction produces the voltage between the telecommunication line conductors which causes interference to the telecommunication signals which is called radio interference. To reduce this radio interference in the telecommunication line, either power line or both power and telecommunication lines are transposed at regular intervals of length the transmission line.
 
6․ In transmission lines distribution constants are
a. resistance               b. inductance and capacitance
c. inductance and resistance           d. both 1 and 2
Explanation::
Transmission line parameters are 1. Inductance 2. Capacitance 3. Resistance 4. Conductance
 
7․ The skin effect of conductor will increase when
a. diameter decrease                   b. resistivity decrease
c. frequency decrease              d. all of the above
Explanation::
Skin effect ∝ 1/skin depth Skin effect is inversely proportional to the skin depth. But, skin depth = 1/√(πfµσ) Where, f = frequency µ = permeability σ = conductivity Therefore, Skin effect ∝ √(πfµσ) When resistivity decrease, its conductivity will increase. This will increase the skin effect.
 
8․ ACSR means
a. Aluminium conductor steel reinforced
b. Aluminium core steel reinforced
c. Aluminium copper steel reinforced
d. all conductor steel reinforced
Explanation::
ACSR means Aluminium Core Steel Reinforced. The inner layer is done by Steel. The outer layer is done by the aluminium. Steel provides the additional mechanical strength.
 
9․ The inductance of single phase two wire line is given by
a. 4×10-7× ln(d/r') H/m           b. 4×10-7× ln(r'/d) H/m
c. 0.4×10-7× ln(d/r') H/m            d. 0.4×10-7× ln(r'/d) H/m
ANSWER: a